The braking equipment or parts of the braking equipment of a motor vehicle are usually operated also in the standstill condition of the motor vehicle in order to maintain the standstill condition of the latter. Generally, for this purpose use is made of a parking brake, which is typically actuated by a vehicle driver, or of braking equipment which in a controlled manner generates braking forces required for the standstill condition.
To start up the motor vehicle, it is necessary to reduce the braking forces generated in the standstill condition.
In the case of conventional parking brakes (hand brakes) which are to be operated by a vehicle driver, it is necessary for the vehicle driver to take not only the measures required for starting up (initial movement or driveaway) but also measures to release the parking brake.
For assisting a vehicle driver during start-up (driveaway) of a motor vehicle it is known from DE-OS-24 20 252 to deactivate the parking brake of a motor vehicle in dependence upon a position of an accelerator pedal. In this case, the parking brake, which in the standstill condition generates a permanently defined braking force, is deactivated to reduce the permanently defined braking force as soon as the accelerator pedal is actuated. This has the result that the parking brake is released even when an actuation of the accelerator pedal does not lead to an driving condition of the motor vehicle which is necessary for driveaway. This is the case, for example, when the vehicle is not started, the engine speed occasioned by the actuation of the accelerator pedal or the engine torque is insufficient to move the vehicle, and the like. Thus, the known parking brake may lead to an unwanted release or to a release of the parking brake which makes it necessary for the vehicle driver during driveaway of the motor vehicle to take additional measures to avoid undesirable driving states.
From DE 36 18 532 A1, and corresponding U.S. Pat. No. 4,717,207, both of which are incorporated by reference herein, driveaway assistance for a motor vehicle on an ascending gradient is known. In this case, a brake system comprises a valve, which is disposed between the master brake cylinder and the wheel brake cylinders and which is closed in the standstill condition to maintain a brake-actuating pressure and opened to cancel the brake-actuating pressure and allow driveaway of the motor vehicle. When the motor vehicle is to be brought from the standstill condition into a driving condition, the engine torque defined by a vehicle driver is checked to see whether it corresponds to a starting torque required for driveaway. In this case, the required starting torque is determined in dependence upon a current angle of inclination of the vehicle on an ascending gradient and upon the vehicle weight. This procedure has the drawback that the current engine torque, even when it corresponds to the required starting torque, does not provide any reliable indication about whether the motor vehicle is actually in a driving state required for driveaway. This is the case, for example, when the clutch is actuated or no gear is selected. When in such cases the required starting torque is reached, the brake system is nevertheless deactivated with the result that an undesirable movement of the motor vehicle may occur.
In the case of this driveaway assistance it is alternatively provided that torques acting upon driving wheels of the motor vehicle be detected. When torques acting upon the driving wheels are high enough to overcome the braking torques generated by the wheel brake cylinders and the backward rolling torque caused by an ascending gradient, the valve is opened to cancel the brake-actuating pressure generated in the standstill condition. This has the drawback that, while the torques of the driving wheels needed to open the valve are sufficient to keep the vehicle stationary particularly on an ascending gradient, they are not high enough actually to bring the motor vehicle from the standstill condition to a driving condition. A further drawback is that this procedure does not check whether the torques acting upon the driving wheels are generated by the engine or are attributable to other causes. Such situations where torques not generated by the engine act upon the driving wheels may arise, for example, when the motor vehicle situated on an ascending gradient is laden.
A general drawback of this driveaway assistance is that the reduction—occasioned by opening of the valve—of braking forces summoned up in the standstill condition is effected independently of the manner in which the motor vehicle is to be moved out of the standstill condition. This may lead to uncomfortable, jerky driveaway phases, during which it may be necessary for the driver to take additional measures to achieve driveaway of the motor vehicle in a desired manner.